A patch, such as a doubler, may be applied to a structure in order to repair a defect or other anomaly in the surface of the structure. Oftentimes, patches must be applied in the field, such as in instances in which a structure is in a remote location or otherwise located some distance from a repair depot. In order to bond a patch to the surface of a structure undergoing the repair, an adhesive, such as a paste adhesive or a film adhesive, is applied between the patch and the surface of the structure.
During repairs performed in the field, the adhesive that bonds the patch to the structure may include voids and, as such, may be more porous than is desired. In this regard, the adhesive may be porous as a result of air that is trapped between the patch and the surface to be repaired and/or degassing of the adhesive. Additionally, a patch that is bonded to the surface of a structure in the field may have a resulting bond line that does not have a consistent thickness, but that, instead, has variations in its thickness that are greater than desired. For example, the bond line thickness may vary as a result of difficulties associated with the maintenance of a constant pressure to the patch during the application of the patch to the underlying structure.
Porosity in a bond line reduces the ultimate bond strength. In instances in which the porosity of the bond line exceeds a predetermined threshold, the patch may be required to be removed prior to treating the surface of the structure and then repeating the process of securing a patch to the surface of the structure with an effort to reduce the porosity of the adhesive with which the patch is secured to the structure. By having to repeat the repair process, the time and cost associated with the repair including the time that the structure is out of service are increased.
Additionally, even in an instance in which the porosity of the bond line satisfies the predetermined threshold, the porosity of the bond line may still exceed that desired for a more permanent repair and/or the bond line thickness may vary more than is desired. As such, upon the return of the structure to a repair depot, the patch that was applied in the field may be removed, the surface of the structure may be treated and another patch may be applied utilizing an autoclave in order to produce a bond line having an acceptable porosity and a consistent bond line thickness. The repetition of the repair in the repair depot may again increase the time and cost associated with the repair and the downtime experienced by the structure.